System and method for remote, free-space optical detection of potential threat agent

ABSTRACT

A system and method for free space, optical remote sensing of a potential threat agent using spectrally responsive sensor material. In one example the sensor material is formed by particles, which in one particular form are porous photonic crystals. The particles are dispersed into an area being monitored for the presence of the potential threat agent. A pair of lasers is used to generate optical light beams that are directed at the sensor particles after the particles have been dispersed. The light reflected by the sensor particles is then analyzed. The presence of the potential threat agent causes a shift in the spectral peak of light reflected from the sensor particles that can be sensed using photo detectors and a processing subsystem. The system can be tuned to remotely detect for specific chemical, biological or environmental agents that may be present within a given area.

FIELD

The present system and method relates to systems for detecting materialthat may pose a chemical, biological or environmental threat to humanhealth or to property. More particularly the present system and methodrelates to an optical system for remotely sensing the presence ofchemical, biological or environmental threat material or otherpotentially deleterious agents.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

The detection of industrial chemical toxicants, chemical warfare agents,environmental toxins, and even explosives, represents an ongoingchallenge. Often, systems that have been constructed for such a purposehave proven to be expensive, relatively complex, or to be otherwiselimited in effectiveness. In addition, when detecting the presence ofhazardous substances within a given area, it would be highly desirableto avoid having an individual enter the area in question even if theindividual is wearing suitable protective clothing. The ability toremotely detect the presence of chemical, biological, explosive andenvironmental agents within a given area, from a remote location, addsan additional degree of assurance to the well being of those individualsoperating such a detection system. Such a system could also have wideranging applicability in the medical and food and/or drug industries.

SUMMARY

The present disclosure is related to a system and method for free space,optical remote sensing for a potential threat agent. The threat agentmay comprise a chemical, biological or environmental material that posesa threat to human health. In one embodiment, the system includes a firstlaser that is tuned to emit optical radiation at a first wavelength. Asecond laser is tuned to emit optical radiation at a second wavelengththat is different from the first wavelength. A sensor agent is placed inan area in which the detection of a potential threat agent may or maynot be present. The sensor agent is constructed having a characteristicsuch that a wavelength of an optical signal reflected by the sensoragent has a spectral peak that closely approximates the firstwavelength, in the absence of the potential threat agent. However, whenthe potential threat agent is present around the sensor agent, lightreflected from the sensor agent will have its spectral peak shifted byat least a small degree. In one embodiment, the shifting of the spectralpeak moves toward the value of the second wavelength.

A subsystem is provided that receives and analyzes optical radiationgenerated by the first and second lasers that is reflected from thesensor agent. The subsystem determines if a shift has occurred in thespectral peak of the optical signal reflected from the sensor agent whenthe sensor agent is disposed in the predetermined area of interest. Ashift of a predetermined magnitude of the spectral peak of the opticalsignal reflected from the sensor agent indicates the presence of thepotential threat agent.

In one embodiment, the subsystem includes first and second detectors,with the first detector receiving the first wavelength optical signalthat is reflected from the sensor agent, and the second detectorreceiving the second wavelength optical signal that is reflected fromthe sensor agent. The intensity of the two reflected optical signals ismeasured by the first and second detectors. A processing subsystemreceives an output from each detector and compares the intensities ofthe two reflected optical signals. From the comparison, a determinationis made as to whether the potential threat agent is present.

In another embodiment a single time-resolved detector is used to receiveboth of the first and second reflected optical signals from the sensoragent. In this embodiment the optical signals generated by the first andsecond lasers are modulated at different frequencies and thetime-resolved detector is synchronized to periodically, alternatelysample both of the reflected optical signals from the sensor agent.

In one embodiment, the sensor agent is comprised of porous photoniccrystals. The crystals are manufactured such that a spectral peak of anoptical signal reflected from the crystals, when the crystals are not inthe presence of the potential threat agent, has a frequencyapproximately equal to the first wavelength of the first optical signal.In another embodiment, the porous photons crystals are made up of poroussilicon photonic crystals.

In one implementation a method is disclosed for performing free space,optical remote sensing for a potential threat agent. The method mayinclude the operations of forming a sensor from particles havingspecific desired spectral characteristics so that optical radiationreflected from the particles has a spectral peak at a firstpredetermined wavelength when the potential threat agent is not presentin a vicinity of the particles, but where the spectral peak shifts whenthe sensor particles are located within a potential threat agent. Firstand second optical beams are used that have different wavelengths toirradiate the sensor particles. The first and second optical beams areanalyzed after they have been reflected from the sensor particles todetermine if the detected intensity of one of the reflected opticalbeams has decreased, while the intensity of the other reflected opticalbeam has increased. When such a condition occurs, this indicates thatthe spectral peak of one of the reflected optical beams has shifted. Theshifting indicates the likely or certain presence of the potentialthreat agent.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a block diagram of a system in accordance with one embodimentof the present disclosure

FIG. 2 is a graph illustrating the representative reflectivity spectraof the sensor crystals in air, and the shift in wavelength of lightreflected from the crystals when the crystals are in the presence ofisopropyl alcohol;

FIG. 3 is block diagram of an alternative embodiment of the system shownin FIG. 1 that makes use of a single time-resolved detector system foranalyzing the reflected optical signals; and

FIG. 4 is a flowchart of various operations that may be performed by thesystem of FIG. 1 in sensing the presence of a potential threat agent.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

Referring to FIG. 1, a system 10 is illustrated for optical, free spaceremote sensing of a potential threat agent. In this example, thepotential threat agent represents a cloud or plume 14 within which asensor agent, represented by particles 12, has been dispersed. As willbe described further in the following paragraphs, the sensor particles12 are constructed with specific, desired spectral characteristics sothat light reflected by each of the sensor particles 12 has a spectralpeak at a predetermined wavelength. The system 10, in the exemplaryembodiment of FIG. 1, includes a first laser 16 and a second laser 18.The first laser 16 generates a coherent first optical beam of lighthaving a first wavelength, for example, 640 nm. The second laser 18generates a second optical beam of coherent light having a differentwavelength from that of the optical beam generated by the first laser16. The wavelength of the light beam generated by the second laser 18,in this example, may be 660 nm. It will be appreciated that theseexemplary wavelengths may be altered as needed for specificapplications, and that the wavelengths of the optical beams generated bythe lasers 16 and 18 will depend at least in part on the spectralcharacteristics that the sensor particles 12 are constructed to have. Itwill be appreciated that in lieu of a pair of lasers 16 and 18, that asingle laser with a tunable emission energy could be incorporated. Thesingle laser could have its laser emission wavelength scanned anddetected over a range of energies that encompass the energies of thespectral peaks of interest.

The first and second optical beams from the first and second lasers 16and 18, respectively, are transmitted through fiber optic cables 20 and22, respectively, to a fiber beam combiner 24. The two optical beams arecombined in the beam combiner 24 and pass through a third fiber opticcable 26 to a collimator 28. The collimator 28 combines the two lightbeams and passes the beams through an iris 30 that further focuses thetwo beams into a single beam. The single optical beam is represented byarrow 32. The single optical beam 32 passes through the center hole of acollection mirror 34 and irradiates at least a subquantity of the sensorparticles 12 dispersed in the cloud 14. In this example, the sensorparticles 12 have been constructed with a spectral characteristic thatcauses light reflected from the sensor particles to have a spectral peakof approximately 640 nm when the particles are disposed in free air (inthe absence of the potential threat agent). The 640 nm wavelength ofreflected light from the sensor particles 12 will thus approximatelymatch the wavelength of the first optical beam generated by first laser16.

It will be appreciated that the particles 12 are also constructed suchthat the presence of a specific chemical, biological or environmentalagent around the particles will cause a known shift in the spectral peakof light reflected by the particles 12. In this example, the cloud orplume 14 may represent isopropyl alcohol vapor that will cause a shiftin the spectral peak of light reflected by the particles 12 towards the660 nm wavelength of the light generated by the second laser 18. It willalso be appreciated that through empirical testing with different threatagents, that a known degree of shifting of reflected light can beexpected. Thus, the lasers 16 and 18 can be selected or tuned to produceoptical beams having particular wavelengths that are intended to detectthe presence of specific threat agents.

With further reference to FIG. 1, the sensor particles 12 may bedelivered into the cloud or plume 14 by any suitable method, such as byan unmanned air vehicle (UAV) or a ground vehicle, or possibly even amarine vessel. In an alternate configuration employing this same opticalinterrogation approach, the sensor particles 12 could be substituted fora sensor material that is not in particle form. For example, a sensormaterial could be deployed in a fixed (non-particulate) configuration,e.g., as an appliqué (e.g., wafer) positioned adjacent a structure suchas a building, dwelling or antenna tower, on a stationary vehicle, oreven carried on the clothing of an individual. In any application, theonly requirement is that the sensor material (i.e., either sensorparticles 12 or some other form of sensor material) belocated/positioned in a manner that enables them to be opticallyinterrogated. In the example shown in FIG. 1, the sensor particles 12could also be delivered into the cloud or plume 14 via a projectile thatreleases the particles 12 at a desired elevation or upon contacting aground surface over which the cloud or plume 14 is present.

Referring further to FIG. 1, an optional arrangement of components toachieve a higher degree of spectral resolution involves directingoptical beams 36 reflected from the sensor particles 12 into acollection mirror 34. The reflected optical beams 36 are then furtherreflected by the collection mirror 34 to another mirror 38. Mirror 38reflects the reflected optical beams 36 through a focusing lens 40 thatfurther focuses the reflected optical beams 36 beams into a focusinglens 42. The focusing lens 42 focuses the two reflected optical beams 36essentially into one focused beam that is made up of the plurality ofreflected optical beams 36. The single reflected beam is denoted byarrows 44. Again, the components 34, 38, 40, 42 and 44 are illustratedmerely to show how the spectral resolution of the reflected opticalbeams 36 could be enhanced.

With further reference to FIG. 1, the single optical beam 44 is directedinto a first bandpass filter 54 that is intended to filter out all lightexcept light of a certain wavelength. In this example, first bandpassfilter 54 is a 640 nm bandpass filter, meaning that only light having awavelength of 640 nm passes through the first bandpass filter 54. Lightoutside of this frequency is reflected by the first bandpass filter 54towards a second bandpass filter 56. The second bandpass filter 56 isselected to have a bandpass wavelength of 660 nm, meaning that onlylight at 660 nm wavelength will pass through it. Arrows 58 represent afirst reflected optical beam that has been filtered to include onlylight having a wavelength of 640 nm. Arrows 60 represent a secondreflected optical beam that has been filtered to include only lighthaving a wavelength of 660 nm.

The optical beam 58 subsequently passes through a lens 62 and impinges afirst photodetector 64. Similarly, optical beam 60 passes through afocusing lens 66 before impinging a second photodetector 68. Each of thephotodetectors 64 and 68 generate electrical output signals that are fedinto an analyzing subsystem 70. The electrical output signal from eachphotodetector 64 and 68 may be represented by the terms V₆₄₀ and V₆₆₀,respectively. The term V₆₄₀ represents the intensity (i.e., magnitude)of the optical beam either incident on detector 64, while V₆₆₀represents the intensity (i.e., magnitude) of the optical beam 60incident on detector 68.

When the cloud or plume 14 (representing the potential threat agent) ispresent around the sensor particles 12, this causes a shift in thespectral peak of light reflected from the particles 12. This shift inthe spectral peak manifests itself by a reduction in the intensity ofthe first reflected optical beam 58 that is sensed by the detector 64.The first detector output voltage V₆₄₀ will then be less than the outputvoltage that would be expected if the sensor particles were present inair or in the presence of random dust particles. Conversely, theshifting of the spectral peak of the reflected light from the particles12 causes an increase in the intensity of the second reflected opticalbeam 60. This increase in intensity is sensed by the second detector 68and indicated by an increase in its output, V₆₆₀, beyond thepredetermined value that would otherwise be produced if the sensorparticles 12 were simply in the presence of air or random dustparticles.

The spectral shifting of the light reflected from the sensor particles12 is further illustrated in FIG. 2. FIG. 2 shows an exemplary waveform72 that represents the light reflected by the sensor particles 12 whenthe sensor particles are simply dispersed in air or within random dustparticles. It will be noted that waveform 72 has a spectral peak with awavelength of about 640 nm. Waveform 74, however, illustrates thereflected light from the sensor particles 12 when the sensor particlesare present within a cloud or plume of isopropyl alcohol (IPA). Thespectral peak of waveform 74 has shifted to about 660 nm.

Returning to FIG. 1, it will be appreciated that during calibration ofthe system 10, the outputs of lasers 16 and 18 may be adjusted inintensity by reflecting their optical beams from isotropic,non-wavelength specific particles, for example, sand (SiO₂). The outputsfrom lasers 16 and 18 may then be adjusted so that the optical signalsdetected by the photodetectors 64 and 68 produce output signals havingthe same magnitude (i.e., so that V₆₆₀ is equal to V₆₄₀).

Referring further to FIG. 1, the processing subsystem 70 may be used toperform a summing operation to examine the output signals V₆₄₀ and V₆₆₀from the photodetectors 64 and 68, to determine if a potential threatagent is present in the cloud or plume 14. When no potential threatagent is present, the output (V₆₆₀) from the second photodetector 68will be less than the output (V₆₄₀) from the first photodetector 64.Thus, in the absence of a specific potential threat agent or random dustparticles, the intensity of the reflected optical beam 58 at the 640 nmwavelength will be larger in intensity than the reflected optical beam60 at the 660 nm wavelength. Thus, the quantity of V₆₆₀ minus V₆₄₀ willbe a negative value, and the negative value indicates to a user of thesystem 10 that the sensor particles are present in free air.

When a specific potential threat agent is present, however, the lightreflected from the sensor particles 12 experiences a shift in itsspectral peak. This cause the intensity (V₆₆₀) of the second reflectedoptical beam 60 to be greater in magnitude than the intensity of thefirst reflected optical beam 58 (V₆₄₀). Thus, the quantity V₆₆₀ minusV₆₄₀ will be a positive value, and such a positive value signifies tothe user that the sensor particles 12 and present within an area wherethe specific potential threat agent is present. When only air or randomdust particles are present in the vicinity of the sensor particles 20,the V₆₆₀ minus V₆₄₀ quantity will be approximately zero. Thus, theprocessing subsystem 70 is able to discriminate whether the sensorparticles 12 are present in a non-threatening atmosphere comprised ofrandom dust particles, present within air, or present within a specificpotential threat agent.

In view of the foregoing, it will also be appreciated while only twolasers 16 and 18 have been illustrated, that a greater or lesserplurality of lasers could be employed as well. The use of two lasers 16and 18 and corresponding photodetectors 64 and 68 forms a “two channel”system that provides a degree of discrimination capability to detectwhether a particular threat agent is present, but also to detect whetherthe sensor particles 12 are located within air or within anothernon-threatening particulate environment. Using a single laser 16 with asingle detector, and with the processing subsystem 70 simply comparingthe output of the detector to a predetermined threshold would stillenable a “Yes”/“No” determination to be made as to whether or not aparticular threat agent is present or not.

It will also be appreciated that the use of three or more lasersgenerating optical beams tuned to three or more wavelengths could alsobe employed to provide an even greater degree ofdiscrimination/detection between different types of threat agents. Sucha multi-channel system could be used to remotely detect for the presenceof a plurality of distinct threat agents from a single remotesite/location.

Referring to FIG. 3, an alternative embodiment of the system 10′ isillustrated that makes use of a single, time-resolved detector system100 and a modulation control subsystem 102. The other components of thesystem 10′ shown in FIG. 3 are otherwise identical in construction andoperation to those shown in FIG. 1, and thus a prime (′) symbol has beenused to denote common components with the embodiment of FIG. 1. Themodulation control subsystem 102 modulates the two lasers 16′ and 18′ attwo different temporal frequencies. The detector system 100 includes asingle time-resolved detector 104 that may be synchronized to the twolaser output signals, for example, alternately every 10 milliseconds, toalternately sample a first reflected optical beam 58′ and a secondreflected beam 60′. The intensities of the sampled, reflected opticalbeams 58 and 60 may then be compared to determine if the potentialthreat agent is present in the vicinity of the sensor particles 12.

Referring now to FIG. 4, a flowchart 200 is illustrated that presents aplurality of exemplary operations that may be performed by the system 10during operation. In operation 202, the sensor particles 12 aredelivered to an area of interest. In operation 204, the first and secondlasers 16 and 18, respectively are used to irradiate the sensorparticles 12 with the first and second optical beams. One or moredetectors (64, 68 or 104) may then be used to receive the reflectedoptical signals 58 and 60 that are reflected from the sensor particles12, as indicated in operation 206. In operation 208, the processingsubsystem 70 is used to examine the outputs of the one more detectors(64, 68 or 104) and to examine the spectral peaks of the reflectedoptical beams 58 and 60. The peak comparison performed at operation 210indicates that if the intensity of the first reflected optical beam isless than the intensity of the second reflected optical signal, then adetermination is made that the potential threat agent is present in thevicinity of the sensor particles 12, as indicated at operation 212.Operations 204-210 may then be repeated as needed, for example, every10-20 milliseconds. If the intensity of the reflected first optical beam58 is greater than the intensity of the reflected second optical beam,then it is determined that the threat agent is not present, as indicatedat operation 214. Operations 204-210 may then be repeated as needed.

The system 10 thus allows for the detection of one or more specificchemical, biological or other environmental agents that may pose athreat to individuals or to property. The system 10 is not limited topotential threat agents that are suspended in air, but could also justas readily be applied to the detection of volatile organic compounds ina liquid. The sensor particles 12 can be implemented as “microscopictags” and used in tracking substances, and substances in liquid form.Other specific applications of the system and method of the presentdisclosure could be in various military and industrial applications toremotely test for the presence of specific, harmful chemical orbiological agents. Still further applications could be in connectionwith the sensing of deleterious volatile organic compounds in thepresence of foods and/or drugs.

While the system 10 is especially well suited for the detection ofcondensable, organic vapors, other methods to induce the shift in thephotonic resonance of the porous photonic crystals that are used to formthe sensor particles may also be used. For example, a shift in thephotonic resonance of the porous photonic crystals could be achieved byplacing the sensor particles 12 in the presence of various otherchemical and biological compounds, that results in chemical reactionscaused by the presence of corrosive species, for example, chlorine,ammonia, or ozone, and catalyzed reactions of chemical agents (e.g., thecatalytic hydrolysis of the nerve agent Sarin to produce HF).

The system and method 10 thus enables the detection of volatilecompounds in air or in water from remote distances using low powerlasers. The system and method 10 provides the advantage that chemical“specificity” can be incorporated into the sensor particle design sothat the system is designed to detect specific chemical or biologicalagents.

While various embodiments have been described, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the present disclosure. The examples illustrate thevarious embodiments and are not intended to limit the presentdisclosure. Therefore, the description and claims should be interpretedliberally with only such limitation as is necessary in view of thepertinent prior art.

1. A system for free space, optical remote sensing for a potentialthreat agent, the system comprising: a laser system for emitting opticalradiation at first and second different wavelengths; a sensor agentlocated remotely from said laser system that is irradiated by said lasersystem, the sensor agent having a characteristic such that a wavelengthof a reflected spectral peak thereof closely approximately matches saidfirst wavelength, in the absence of said potential threat agent, andsuch that said reflected spectral peak changes in wavelength from saidfirst wavelength when said sensor agent is exposed to potential threatagent; and a detector system that receives and analyzes opticalradiation generated by said laser system that is reflected from saidsensor agent, to determine if a shift in said reflected spectral peakaway from said first wavelength has occurred, indicating a presence ofsaid potential threat agent.
 2. The system of claim 1, wherein saidlaser system comprises first and second lasers, with said first laserbeing tuned to emit optical radiation at said first wavelength, and saidsecond laser tuned to emit optical radiation at said second wavelength.3. The system of claim 1, wherein said detector system determines ifsaid reflected spectral peak has shifted away from said first wavelengthtoward said second wavelength.
 4. The system of claim 3, wherein saiddetector system includes: a first detector for detecting an amplitude ofa first reflected spectral peak at said first wavelength; a seconddetector for detecting an amplitude of a second reflected spectral peakat said second wavelength; and a system for processing said detectedamplitudes to determine if said amplitude of said first reflectedspectral peak has been reduced in magnitude while said amplitude of saidsecond reflected spectral speak has increased in spectral magnitude,thus indicating the presence of said potential threat agent.
 5. Thesystem of claim 4, wherein: said first detector includes a firstbandpass filter that passes only optical radiation having said firstwavelength; and said second detector includes a second bandpass filterthat passes only optical radiation having said second wavelength.
 6. Thesystem of claim 1, wherein said sensor agent comprises porous photoniccrystals.
 7. The system of claim 1, wherein said first and second lasersare modulated at different temporal frequencies.
 8. The system of claim1, wherein said system for processing said detected amplitudes comprisesa single time-resolved detector.
 9. A method for free space, opticalremote sensing for a potential threat agent, the method comprising: a)forming a sensor having a characteristic such that optical radiationimpinging the sensor and reflected from the sensor has a spectral peakwith a first wavelength; b) directing a first optical beam having saidfirst wavelength at said sensor; c) directing a second optical beamhaving a second wavelength at said sensor; d) analyzing said first andsecond optical beams after said beams have been reflected from saidsensor to determine if one of said optical beams has experienced aspectral shift; and e) interpreting said spectral shift as indicatingthe presence of said potential threat agent.
 10. The method of claim 9,wherein operations a) and b) comprise using a pair of lasers modulatedat first and second temporal frequencies, to generate said first andsecond optical beams, respectively.
 11. The method of claim 9, whereinoperations a) and b) comprise using first and second lasers thatsimultaneously generate said first and second optical beams.
 12. Themethod of claim 9, wherein operations a) and b) comprise using a singlelaser with a tunable emission energy, where the laser emissionwavelength is scanned and detected over a range of energies thatencompass the energies of the first and second peaks of the sensor. 13.The method of claim 9, wherein operation d) comprises using a singletime-resolved detector to receive said first and second optical beamsafter said beams have been reflected from said sensor.
 14. The method ofclaim 9, wherein operation d) comprises using separate detectors tomeasure an amplitude of each of said optical signals.
 15. The method ofclaim 9, wherein operation a) comprises forming a sensor from poroussilicon photonic crystals.
 16. A method for free space, optical remotesensing for a potential threat agent, the method comprising: a) forminga sensor from spectrally responsive sensor material having acharacteristic such that optical radiation impinging the sensor materialand reflected from the sensor material has a spectral peak at apredetermined wavelength, when said potential threat agent is notpresent in a vicinity of said sensor material; b) using first and secondoptical beams having different wavelengths to irradiate said sensormaterial; and c) analyzing said first and second optical beams aftersaid beams have been reflected from said sensor material to determine ifsaid spectral peak of one of said reflected optical beams has shifted,said shifting indicating the presence of said potential threat agent.17. The method of claim 16, wherein operation a) comprises forming saidsensor material from porous photonic crystals.
 18. The method of claim14, wherein operation a) comprises forming said sensor material fromporous silicon photonic crystals.
 19. The method of claim 16, whereinoperation c) comprises receiving said first and second optical beamsreflected from said sensor material with separate detectors.
 20. Themethod of claim 19, wherein operation c) further comprises using a pairof filters to provide said reflected optical beams to said detectors.